Make the foreach early-exit semantics consistent

[eddyashton]

Noticed as part of #1221 (and one need manually merging when the other goes in, due to similar changes in tx_view.h):

TxView::foreach takes a visitor lambda, and returning false is supposed to mean "I'm done, don't call me again". We need to run this twice, for both the original state and the current write set. The early-out-on-false behaviour only applied to the writes - if you decided you were done after looking at something in the state, you still get called with something in the write set. This fixes that - return false means you won't get called again at all.

On a related point, foreach itself returned a bool with no meaning. It could return "the lambda returned false at some point", or "you saw everything", but none of these seem particularly clear or useful. For clarity, I've just removed the return type entirely.

++unit tests

[ghost]

foreach_tests@8766 aka 20200528.6 vs master ewma over 50 builds from 8553 to 8761

[achamayou]

Nice catch!